Computing devices can exchange digital data through communications transmitted across computer networks to which such computing devices are communicationally coupled via network interface hardware. Traditionally, to communicate digital data from a sending computing device to a receiving computing device across a network, network interface hardware of the sending computing device receives a copy of the digital data to be transmitted, and then transmits such digital data, in accordance with relevant network communication protocols, to the receiving computing device, whose network interface hardware receives the digital data, a copy of which can then be further made, on the receiving computing device, in order to enable processes, executed by a general-purpose central processing unit of such a receiving computing device, to access the received data. Consequently, the transmission of digital data can involve the making of multiple copies of such data, both at the sending computing device, and at the receiving computing device.
Remote direct memory access communications can enable a sending computing device to transmit data without first making a copy of such data for the network interface hardware on such a sending computing device. Instead, remote direct memory access communications can enable the network interface hardware of a sending computing device to access data directly from memory in which such data was stored by an application program seeking to transmit such data across the network. Remote direct memory access communications can also enable the network interface hardware of the sending computing device to write data directly into the memory of a receiving computing device, via the network interface hardware of the receiving computing device, but without making an additional copy on the receiving computing device.
Remote direct memory access communications rely on registered memory due at least in part to the direct reading of data out of memory by the network interface hardware of the sending computing device and the writing of data directly into the memory of the receiving computing device by the same network interface hardware. Because of such direct reading of data, the underlying memory cannot have its contents changed by other processes. However, registered memory cannot be paged to disk and, as such, ties up physical Random Access Memory (RAM) that can be at a premium in certain systems. Consequently, remote direct memory access communications only register small quantities of memory for send and receive buffers, and then slice up larger quantities of data to fit such smaller buffers, and then transmit the slices one at a time. Larger buffers can be registered to transmit larger quantities of data at one time, but such registration processes take so long that efficiencies gained by transmitting the larger quantities of data in a single communication are negated by the length of time taken to register the buffers to do so.